(maint) Use LOGDIR in Dockerfile
(maint) Use LOGDIR in Dockerfile

 - Remove erroneous note about ENV vars being able to use other ENV
   vars. An ENV var can modify another ENV var as long as the values
   are not set in the same block.

   Separate out the use of LOGDIR to a separate ENV block so that it
   can be consumed in PUPPETDB_JAVA_ARGS.

 - Also use $LOGDIR in the later mkdir -p

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb